In this paper we present a novel and original four channel front-end developed for a beam position monitor (BPM) system. In this work, we demonstrate for the first time the continuous calibration of the system using a pilot tone for both beam current dependency and thermal drift compensation, eliminating the need for thermoregulation. By using this original approach, we were also able to investigate several odd and well-known behaviours of BPM systems; the influence of important issues, like the non-linearity of ADCs and the gain compression of amplifiers which do affect the reliability of the measurement, have been fully understood. To achieve these results, we developed a new radio frequency front-end that combines the four pick-up signals originated by the beam with a stable and programmable tone, generated within the readout system. The signals from a button BPM of Elettra storage ring, have been acquired with a 16 bit - 160MS/s digitizer controlled by a CPU that evaluates the acquired data and applies the correction factor of the pilot tone. A final resolution equal to 1.0um, on a 20mm average radius vacuum chamber, has been measured with a long-term stability less than 1um.